The long-term goals of these studies are to understand regulation of expression of glial cell line-derived neurotrophic factor (GDNF) in skeletal muscle and to determine the consequences of altered skeletal muscle GDNF expression with age. If GDNF expression in skeletal muscle is controlled by physical activity, then decreased physical activity, which is commonly observed in aging individuals, will lead to decreased GDNF expression. Decreased GDNF supply could result in alterations in motor neuron structure and function. Three specific hypotheses are proposed. Hypothesis 1.) Aging and inactivity (hindlimb unloading) decrease GDNF expression in skeletal muscle. Hypothesis 2.) Exercise training restores GDNF expression in skeletal muscle of hindlimb-unloaded rats and aged rats. Hypothesis 3.) Electrical activity and mechanical activity are both important regulators of GDNF expression in skeletal muscle. Fisher 344 rats will be used for all studies. Levels of mRNA for GDNF will be measured using real-time PCR, levels of GDNF protein will be measured using enzyme-linked immunosorbant assay and distribution of GDNF protein and patterns of motor innervation will be examined using immunocytochemical methods. The effects of mechanical and electrical activity on GDNF expression will be examined in isolated skeletal muscle preparations in muscle bath studies. We expect the results to show that GDNF expression in skeletal muscle decreases with hindlimb unloading and decreases with age. We predict that walk-training will restore expression of GDNF in skeletal muscle from hindlimb-suspended rats and skeletal muscle from aged rats. The final set of studies will show that both electrical and mechanical activity are important in regulating the level of expression of GDNF in skeletal muscle. A better understanding of the relationship between physical activity, GDNF expression and motor innervation, will aid in understanding how these processes become disturbed with aging. Findings of these studies may help to identify potential sites for therapeutic intervention to help slow or reverse changes occurring with aging, injury or disease.